The present invention generally relates to a spread spectrum communication system, and more particularly to a spread spectrum communication system having a transmitter for transmitting a spread spectrum signal modulated by two data signals and having a receiver for receiving the spread spectrum signal to reproduce the data signals by a synchronization loop.
Recently, several types of spread spectrum communication systems have been proposed for radio communication, digital data communication, local area network communication and cordless telephone systems.
"Coherent Spread Spectrum Systems" page 347 by J. K. Holmes, published by R. E. Krieger Publishing Company of Florida in U.S.A. (reprint edition 1990), discloses a quadriphase direct sequence transmitter for spread spectrum communication in which a quadriphase pseudonoise (PN) signal to be transmitted is generated by modulating two distinct codes (having a small crosscorrelation) in accordance with two data signals.
"Spread Spectrum Communication System In ETS-VI Inter-Satellite Communication Experiments" by H. Kikuchi et al., a transaction of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan, SSTA90-45, Oct. 25 and 26, 1990, discloses an experimental spread spectrum communication system to which the above described Holmes method is used.
FIG. 1A shows a modulator of the experimental spread spectrum communication system disclosed in the above mentioned publication. This spread spectrum modulator carries out the pseudonoise signal modulation of the S-band Inter-satellite Communication (SIC) return link. FIG. 1B shows a demodulator of another experimental spread spectrum communication system disclosed in the above mentioned publication. This spread spectrum demodulator carries out the pseudonoise signal demodulation of the SIC return link.
The spread spectrum modulator in FIG. 1A includes an oscillator (OSC) 41 for generating a clock signal, a synthesizer 42, an I-channel pseudonoise (PN) generator 43, a Q-channel pseudonoise (PN) generator 44, a first convolutional coder 45 for encoding I-channel data to produce a first code sequence, a second convolutional coder 46 for encoding Q-channel data into a second code sequence, four multipliers 47a through 47d with two .pi./2 phase shifters, and an adder 48.
In FIG. 1A, a clock signal from the OSC 41 is supplied to the synthesizer 42, and the synthesizer 42 generates a carrier signal and a drive clock signal, in accordance with the clock signal from the OSC 41. The drive clock signal from the synthesizer 42 is supplied to each of the PN generators 43 and 44, so that the PN generators 43 and 44 respectively generate a first PN signal and a second PN signal in accordance with the drive clock signal from the synthesizer 42. The coder 45 produces the first data signal (code sequence) in accordance with the I-channel data, and the coder 46 produces the second data signal (code sequence) in accordance with the Q-channel data.
The first data signal from the coder 45 is multiplied at the multiplier 47a by the first PN signal from the generator 43 to produce a first spread spectrum signal, and the second data signal from the coder 46 is multiplied at the multiplier 47b by the second PN signal from the generator 44 to produce a second spread spectrum signal. The carrier signal and a .pi./2 phase-shifted carrier signal are multiplied at the multipliers 47c and 47d by the first and second spread spectrum signals from the multipliers 47a and 47b, respectively. The two resulting signals from the multipliers 47c and 47d are added to each other at the adder 48 to produce a SIC return link signal.
The spread spectrum demodulator in FIG. 1B includes two multipliers 51a and 51b with a .pi./2 phase shifter, a PN acquisition unit 52, a carrier reproducing unit 53, a PN tracking unit 54, an I-channel PN generator 55, a Q-channel PN generator 56, two clock units 57a and 57b, an I-channel demodulator 58a, a Q-channel demodulator 58b, and two Viterbi decoders 59a and 59b.
In the demodulator in FIG. 1B, first and second spread spectrum signals of intermediate frequency (IF) are produced by using two carrier signals with a .pi./2 phase difference. The autocorrelation between the first spread spectrum signal and the first PN signal is obtained to produce a first correlated signal, and the autocorrelation between the second spread spectrum signal and the second PN signal is obtained to produce a second correlated signal. By using the difference between the first and second correlated signals, a sync signal of the PN signals is generated. The demodulated signals in a narrow band of frequencies are produced by multiplying the IF spread spectrum signals by the synchronized PN signals, and the carrier signal component is eliminated from the demodulated signals, so that the I-channel and Q-channel data is reproduced.
"Spread Spectrum Communication System" page 280 by M. Yokoyama, published by Kagaku Gijutsu Publishing Company of Japan on May 20, 1988, discloses a Costas-loop type spread spectrum communication system in which a synchronizing demodulator called the Costas loop is used. This synchronizing demodulator is a composite phase locked loop for demodulating a suppressed carrier signal, and it has as input a frequency modulated (FM) signal or a phase modulated (PM) signal, and the carrier reproduction and the data signal demodulation are carried out by using the input signal.
The conventional spread spectrum communication systems mentioned above are of this type, and it is necessary that each of the transmitter and receiver in the system of this type has a phase shifter, a power synthesizer, a power distributor and two PN generators. Thus the conventional systems require a complicated structure, and the manufacturing cost becomes high.
FIG. 2 shows a typical carrier reproducing circuit in which a modified Costas loop is used. This carrier reproducing circuit carries out multilevel quadrature amplitude modulation (QAM). The carrier reproducing circuit in FIG. 2 includes two multipliers 61a and 61b, a .pi./2 phase shifter 62, two multilevel discriminators 63a and 63b, two exclusive-OR circuits (EXOR) 64a and 64b, a difference circuit 65, a low-pass filter (LPF) 66, and a voltage-controlled oscillator (VCO) 67.
As described above, in order to transmit two-channel information signals with the conventional spread spectrum communication systems described above, it was necessary that each of the transmitter and the receiver has two PN generators which generate two kinds of PN signals with a small crosscorrelation. In order to reduce the interference between the two PN signals modulated by the two-channel information, it was necessary that two carrier signals with a .pi./2 phase difference are used. There is another conventional spread spectrum communication system which uses the two carrier signals with a .pi./2 phase difference. However, there still remain the problems in that both the transmitter and the receiver of the conventional system require two or more PN generators to produce a signal to be transmitted, and that the radio frequency part of the system requires the phase shifter, the power synthesizer and the power distributor. Thus, the conventional systems described above require a complicated structure, and the manufacturing cost becomes high.